That's what I was thinking, except then I saw that the half armor list includes the peascod cuirass, which gives a higher DR than most medium and full armor for less money. That made me wonder if I was missing something.

The difference between Codex armor rules and SRD is that in the SRD, the armor is always the same coverage, so they are just breaking it down by effectiveness of the armor and bulk of the armor. (in the 3.5 players handbook the illustration of the 'breast plate' was an entire suit of armor covering the legs and arms as well as the torso).

In the Codex you also have coverage (bypass) so that is another really important factor.

For example a helmet has a very high DR but they bypass is easy (like -1 IIRC).